Wireless transmission devices in wireless communication systems are provided with a power amplifier (hereinafter sometimes referred to as a “PA”) that amplifies the power of a transmission signal. The wireless transmission devices generally operate the PA in or near the saturation region of the PA to increase the power efficiency of the PA. However, when the PA is operated in or near the saturation region, nonlinear distortion increases. Thus, in order to reduce the nonlinear distortion, that is, in order to improve the Adjacent Channel Leakage Ratio (ACLR), the wireless transmission devices are provided with a distortion compensator that compensates for nonlinear distortion.
One distortion compensation method used in distortion compensators is a “predistortion method.” Hereinafter “predistortion” is sometimes referred to as “PD.” A distortion compensator using the PD method multiplies in advance a signal before input to a PA by a distortion compensation coefficient having an inverse characteristic of nonlinear distortion of the PA, to increase the linearity of the output of the PA and suppress distortion of the output of the PA. A signal multiplied by a distortion compensation coefficient is sometimes called a “predistortion signal (PD signal).” Therefore, a PD signal is a predistorted signal according to an inverse characteristic of nonlinear distortion of a PA before being input to the PA.
For example, some distortion compensators using the PD method have a “distortion compensation table” in which a plurality of distortion compensation coefficients are stored, and read from the table a distortion compensation coefficient that depends on the amplitude value of a baseband signal input to the distortion compensators, to multiply the baseband signal by it. Hereinafter, distortion compensation performed using a distortion compensation table is sometimes referred to as “look-up table (LUT)-type distortion compensation.”
Some distortion compensators using the PD method approximate an inverse characteristic of nonlinear distortion of a PA by a “power series,” and perform distortion compensation using the power series. Hereinafter, distortion compensation performed using a power series is sometimes referred to as “series-type distortion compensation.” A distortion compensator using the series-type distortion compensation generates a plurality of higher-order signals on a baseband signal input to the distortion compensator, multiplies the higher-order signals by a distortion compensation coefficient prepared for each order, and synthesizes all the signals multiplied by the distortion compensation coefficients to generate a PD signal.
Examples of related-art are described in Japanese Laid-open Patent Publication No. 2007-288492, in Japanese Laid-open Patent Publication No. 2009-200694, in Japanese Laid-open Patent Publication No. 2010-258932, and in International Publication Pamphlet No. WO 2012/111583.
Further, examples of related-art are described in H.-H. Chen, C.-H. Lin, P.-C. Huang, and J.-T. Chen, “Joint Polynomial and Look-Up-Table Predistortion Power Amplifier Linearization,” IEEE Transactions on Circuits and Systems II, Express Briefs, vol. 53, no. 8, pp. 612-616, August 2006.
FIGS. 1 and 2 are diagrams for explaining a problem. In a case where a baseband signal is a “multicarrier signal” including a plurality of signals of different carrier frequencies, when a PA is operated in a nonlinear region on the multicarrier signal, intermodulation distortion (hereinafter sometimes referred to as “IM”), which is nonlinear distortion, can occur. For example, as illustrated in FIG. 1, when a multicarrier signal including a signal of a carrier frequency f1 and a signal of a carrier frequency f2 is amplified in the nonlinear region, a signal output from the PA can include IM3, IM5, which are IM of third-order, fifth-order, respectively. Hereinafter, the signal of the carrier frequency f1 is sometimes referred to as a “carrier signal 1,” and the signal of the carrier frequency f2 as a “carrier signal 2.” The IM occurs in contact with both sides of the carrier frequencies f1, f2, and also occurs at frequencies at predetermined distances by “Δf=|f1−f2|” from the carrier frequencies f1, f2 on the frequency axis as illustrated in FIG. 1. The IM3, IM5 that occurs at the frequencies at the predetermined distances from the carrier frequencies f1, f2 occur symmetrically on the frequency axis when viewed from a center frequency f0 of the multicarrier signal. The center frequency f0 of the multicarrier signal including the carrier signal 1 and the carrier signal 2 is “f0=(f1+f2)/2.”
The bandwidth of a “digital signal band” illustrated in FIG. 1 is defined according to a sampling rate of a digital baseband signal. A digital signal band normally corresponds to a distortion compensation range in a distortion compensator.
Hereinafter, IM that occurs in contact with carrier frequencies is sometimes referred to as “first IM,” and IM that occurs at frequencies at predetermined distances from the carrier frequencies as “second IM.”
Since a distortion compensation range spans the entire digital signal band as illustrated in FIG. 1, the distortion compensator compensates for both of the first IM and the second IM collectively. Therefore, the distortion compensator performs uniform distortion compensation on all the IM, using a distortion compensation coefficient that takes into consideration all the IM existing in the digital signal band, that is, an average distortion compensation coefficient for all the IM. Thus, all the IM is averagely and uniformly suppressed, so that the first IM is not suppressed sufficiently, and as illustrated in FIG. 2, the first IM can remain. When the first IM remains, the ACLR is degraded.